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What exactly are the main voltage standards called for all the common voltages we see? 3.3V, 5V and 12V are some of the most common DC voltages but what are they known as exactly and what's with their significance? i.e. Were they picked as it sounded like a nice value or for an electronically significant reason?

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marked as duplicate by tcrosley, Ricardo, PeterJ, Adam Lawrence, placeholder Oct 26 '14 at 5:40

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  • \$\begingroup\$ See also E series en.m.wikipedia.org/wiki/Preferred_number but that should mean 5.1V instead of 5V. I have seen, but it is unusual, power supplies specified as 5.1V. \$\endgroup\$ – Martin Oct 25 '14 at 21:36
  • \$\begingroup\$ @Martin You often see UBECs rated at 5.1V. \$\endgroup\$ – Majenko Oct 25 '14 at 22:02
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3.3 volts was chosen as a JEDEC standard. Electronics had moved on from the the old 5V (TTL) and 4000 series CMOS (3 - 15V).

Technology changed and industry moved toward a lower voltage:

e.g. Transistors got smaller, so the threshold voltage is lowered, a need for faster circuits, lowering the voltage will reduce the time it takes to change logic levels, TTL logic is being phased out, less demand for 5V compatibility higher packaging density so power usage is a concern and so on.

As a world wide industry agreed standards are important to ensure product compatibility.

The 12V probably comes from the use of battery power - most cars using a '12V' lead acid accumulator. Trucks tends to use 24V and aviation has its own standards. As far as I am aware 12V was never an electronics standard, it was simply in common use as was (is) 9V.

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5V is commonly known as TTL logic level voltage, 3.3V is commonly known as CMOS logic level voltage. I'm not aware of any special name for 12V

The TTL logic level voltage was set at 5V because that was the lowest voltage they could afford with the current consumption of the device to allow manufacturability and noise proof performance up a certain standard they were after and thought of as acceptable. This is simply what TTL allowed back in the day. With CMOS the voltage could drop and this lowered power consumption, among other things. Today CPUs are powered by even lower voltages in the 1+V range.

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    \$\begingroup\$ This is not correct. "CMOS logic" does not imply 3.3 V, it implies that the switching threshold is at roughly 0.5*VCC. CMOS logic parts are available with operating voltages from 1.2 V to 24 V. \$\endgroup\$ – The Photon Oct 25 '14 at 22:23
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    \$\begingroup\$ There is a correspondance between 5V TTL (bipolar) and 3.3V. Because of the nature of bipolar transistors, 5V TTL use asymetric levels : High [5V .. 2.4V] and Low [0.8V .. 0V]. CMOS instead can be more easily balanced, and 3.3V CMOS can have the same logic transition levels as TTL gates and are therefore compatible. Now, voltages are decreasing so high speed components and interfaces use lower voltages. For example : SDRAM=3.3V, DDR2=1.8V, DDR3=1.5V... etc.. \$\endgroup\$ – TEMLIB Oct 25 '14 at 23:33
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    \$\begingroup\$ @ThePhoton Please read what I've wrote again, I did not say that CMOS works exclusively at 3.3V, however 3.3V voltage standard for interfacing is normally referred to as CMOS logic level voltage. I agree the term might not be spot on accurate. \$\endgroup\$ – user34920 Oct 26 '14 at 9:51
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This is not about the 3.3V and 5V standard, but about the others you see - 12V, 9V, etc.

Common alkeline battery cells have a natural voltage of about 1.5V. That's not so much by design as purely how the chemistry works.

So making a battery by combining cells you naturally end up with multiples of 1.5.

6 cells together makes a 9V battery. 8 cells together makes a 12V battery.

Car batteries (nominal 12V) have a higher voltage per cell, so 6 individual lead acid cells (pairs of plates) in an acid solution make up a 12V (higher O/C) battery. The same with 12V sealed lead-acid batteries.

Different chemistries give slightly different voltages, so you will see 1.2V (NiMh), 2.1V (lead acid), 3.6V (lithium) etc.

Lithium Ion and its derivatives, which are a completely different kettle of fish, of course, have the highest of all the common per-cell voltages, and devices powered by those will have voltages in multiples of ~3.6V.

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    \$\begingroup\$ The cells in a lead-acid battery DO NOT share a common fluid bath. a "12 volt" car battery has six separate compartments, one for each cell. If the cells shared the electrolyte container there would be short circuits between the cells. \$\endgroup\$ – Peter Bennett Oct 25 '14 at 22:08
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    \$\begingroup\$ It's late here and I'm not thinking straight. Of course they're separate - why else would they have multiple refill points... \$\endgroup\$ – Majenko Oct 25 '14 at 22:11

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